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Old Mar 20, 2007, 10:02 PM   #11
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Focal Length: 30mm
ISO: 400
Aperture: F13
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Old Mar 20, 2007, 10:02 PM   #12
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Focal Length: 30mm
ISO: 400
Aperture: F16
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Old Mar 20, 2007, 10:03 PM   #13
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Focal Length: 30mm
ISO: 400
Aperture: F19
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Old Mar 20, 2007, 10:03 PM   #14
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Focal Length: 30mm
ISO: 400
Aperture: F22
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Old Mar 20, 2007, 10:05 PM   #15
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Focal Length: 30mm
ISO: 400
Aperture: F27
Shutter speed was about 1/20s for this one so might not be accurate due to it being hand held shot.
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Old Mar 21, 2007, 2:50 AM   #16
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Also, for comparison I took a couple of photos of the same shot, using a tripod same exposure settings with another lens, wasn't quite the exact same focal length however.

70mm
1/90s
F5.6
ISO400

Shot on the left is Sigma 18-125mm F3.5 - 5.6 (the one used in the rest of pics)
Shot on the right is Sigma 70-300mm APO F3.5 - 5.6

difference seems pretty clear in terms of clarity, even though the right side is slightly further zoomed in, i wouldn't expect that to account for such a difference in sharpness.
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Old Mar 21, 2007, 4:26 AM   #17
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The first series shows quite nicely how changing the aperture affects the quality however it is the comparison shot that is interesting and to me it looks as if the 18-125 missed focus rather than being soft. Did you take more than one shot at this setting to ensure it was not just a chance missed focus? It could be there is a front/back focus problem with the lens. You are right though it is not the amount of zoom that is making the difference in sharpness so is either bad glass or focus.

If you want to check the focus there are numerous options, here is a like to one which should get you started.

http://photo.net/learn/focustest/
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Old Mar 21, 2007, 4:38 AM   #18
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Let'spresume that all the shots were taken from the same distance and camera shake is not an issue. From examining the texture of the mortar, and the edges of the bricks, it seems your lens is sharpest from f/5.6 to f/13, peaking at about f/8-f/9.5. I presume that these shots are from the centers of the original images, and I would guess that similiar crops from the edges would be less sharp.

"compare.jpg" doesn't help much since the magnifications are different. While the brick on the left is only about 390 pixels tall, the brick on the right is about 475 pixels tall, so (presuming that all the bricks are the same size) the brick on the right would naturally appear sharper. I don't think we can draw any conclusions from this image.

It seems that, for maximum sharpness,you shouldn't use this lens with anaperaturesmaller than f/13, at least at a focal length of 30mm. Your original shot was at 34mm, so I think we've demonstrated that it would be soft at f/19.
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Old Mar 21, 2007, 4:48 AM   #19
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Mark1616 wrote:
Quote:
The first series shows quite nicely how changing the aperture affects the quality however it is the comparison shot that is interesting and to me it looks as if the 18-125 missed focus rather than being soft. Did you take more than one shot at this setting to ensure it was not just a chance missed focus? It could be there is a front/back focus problem with the lens. You are right though it is not the amount of zoom that is making the difference in sharpness so is either bad glass or focus.

If you want to check the focus there are numerous options, here is a like to one which should get you started.

http://photo.net/learn/focustest/
I did perform about 5 shots of each lens and they were all basically the same, so I don't think it's the focus but I will certainly take a look at that link and give it a shot.
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Old Mar 21, 2007, 9:13 AM   #20
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Another strange thing has happened, in the course of fiddling around and testing with the lens, it seems to have dramatically improved for no apparent reason.
The 2 photo crops below were both taken with the same exposure settings, same focal length, my position seems to be slightly different between the 2, but the variance in clarity is pretty big I think. The top half is after things improved, bottom is before. I've taken quite a few photos both sets so I don't think the difference is camera shake. The clarity around the pocket bag is particularly noticable, also around the yellow ball itself, which was incidently the focal point for both shots. So now I'm wondering whether there's some kind of intermittent problem with either camera or lens...
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